texts: Can a binary star optically "orbit" a planet?

texts:
I was watching a science fiction anime from the mid 70's. In it, there was a star orbiting a planet. Since this seemed impossible, and I had never heard of it, I looked on the internet to see if this could be possible in a very unlikely situation or in theory.

On a similar question on Quora, after many no, no, no, no answers from astronomers/astrophysicists, some of them with very detailed models and theories with different scenarios and equations, one answer read,

  Absolutely.

  If there were two stars of identical mass orbiting each other in equilibrium, then you could throw a planet of any size at the barycenter of the two stars. Then optically you'd have not just one, but two stars orbiting a planet. The planet would be equidistant from the two stars at all moments in time, and would experience identical gravitational pull in two diametrically opposite directions and hence never deviate from the barycenter. This system would not be in a stable equilibrium though, and a passing celestial object of any mass could disrupt this system. Hence, this is probably something that is unlikely to be observed.

Is this possible?


texts: How do astronomers find out the correct weight of a planet even though there isn't any direct means to weigh them? What technique do they use? Just curious!


texts: How are space telescopes stabilised to a perfect standstill?

texts: I assume that for space telescopes to get good images, they must be put in perfect standstill, as even the slightest deviation in the viewing angle is amplified by the distance of observed objects, results in observing a different area of space. How is this feat accomplished? How can you stabilize an instrument in space, so that it can take images of galaxies gazillion light years away? And how do you know you have succeeded, apart from getting quality imagery?


texts: What's the brightness of Alpha Centauri from Proxima Centauri?

texts: Self-explanatory, but I would like a comparison as well. Is the light enough to see by? How disrupted will the pitch darkness on the spot opposite of the 'solar pole' be?


texts: How do astronomers detect gases that are in the atmosphere of exoplanets?

texts: Exoplanets are planets that are located outside our solar system - whether that be orbiting a star or drifting past one. Now, the closest star to us is Alpha Centauri which is just over four light-years away. So, how can astronomers detect gases in the atmospheres of planets over this distance with any degree of accuracy (or at all)?


texts: Binary pairings that haven't been discovered yet?

texts:
Question: Are there any kinds of binary pairings that haven't been discovered yet? Any that are particularly significant, or that might shed some light on binary stellar evolution or theories of capture processes, that are sought, but so far no examples have been found or at least suspected?

By "pairing" I mean a binary object where each is a type of star or a black hole.

Stars can be anything from brown dwarfs to neutron stars. Black holes should be very roughly comparable to stars in size. (I'm not asking about a star in orbit around a supermassive black hole in the center of a galaxy)

For example, recent observations of gravitational wave events have suggested a pair of black holes merging, and a pair of neutron stars merging.


texts: Estimates for how many Trans-Neptunian Objects there are

texts:
The Minor Planet Center shows that we know of about 1350 Trans-Neptunian Objects (TNO's).  I think it is safe to assume that we have not found all the TNO's there are to find, even the ones that we are capable of detecting with current telescope technology (simply because we haven't looked at them yet).  Are there any estimates for how many TNO's exist (not just how many we've discovered)?

I know such estimates probably are for numbers of objects with a radius greater than a certain amount and for objects belonging to a certain group (Plutinos, Kuiper Belt Objects, scattered disk objects, etc.).  I am fine with estimates that make such size cuts and group specifications.


texts:
I work with astrophysicists and require some basic knowledge of many astronomical sources, however research priorities often demand that most of human knowledge on a subject be taken for granted.

I am currently researching galactic novae, and I find it relevant to summarize their history briefly when presenting my research to certain audiences. Unfortunately, I am unable to find any source material which describes how we know one key aspect of the events: that they are an accreting white dwarf in a stellar binary. This fact appears to be so well founded that no scientific paper feels obligated to cite it when stated, but basic resources like astronomical encyclopedia also make no reference that I've seen.

How do we know that novae are binary systems?

E.g., have follow-up observations clearly identified the white dwarf and its companion? Or do other astronomical measurements strongly confirm this binary hypothesis (and make it all but obviously true)? I apologize if it's as simple as "someone looked through a telescope, and it was pretty obvious" -- in my experience no revelation in astrophysics is nearly so simple, but certainly this could be the case.


texts: If 50 tons or more of debris falls to earth everyday, is Earth getting heavier?

texts: In turn, would that increase our gravitational pull?


texts: How hot must a star get before it is considered to be a star?

texts: How hot must a star get before it actually becomes a star? Why does it need to get so hot? Please find an official site to quote from, if you can.


texts: How does an absolute horizon form before the apparent horizon?

texts:
If I understand it correctly, the apparent horizon is the boundary between where light directed outwards will move outwards, and light directed outwards will move inwards. Even if light is moving away from the black hole outside of the apparent horizon, it will not escape to infinity if it is within the event horizon.

The event horizon is essentially the apparent horizon in the future, if the apparent horizon continues growing. Theoretically, the two horizons should eventually meet up...again, if I understand it correctly.

My confusion is with how the two form. According to Wikipedia,

  In the simple picture of stellar collapse leading to formation of a black hole, an event horizon forms before an apparent horizon.

My main question is, how can the event horizon form "before" the apparent horizon? I understand that one can slice the Schwarzschild geometry so that there is no apparent horizon at all, but if an apparent horizon exists, I still don't understand how or why it would form after the event horizon.


texts: Why does lithium fuse at lower temperatures than hydrogen?

texts:
This is a basic question, but it's been bugging me. In the Wikipedia article for lithium burning, it states that:

  Stars, which by definition must achieve the high temperature (2.5 × 10^6 K) necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of lithium-7 and a proton producing two helium-4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion.

I would imagine that with lithium having more protons, it would have a stronger Coulomb repulsion and require higher temperatures to fuse with hydrogen. Now, this article is pretty sketchy since it cites no sources, and I would normally dismiss this. But according to here, lithium burning occurs in protostars, before hydrogen fusion even takes place. How could lithium fuse with hydrogen at temperatures that low?


texts: How would one calculate the synodic period between 3 planets?

texts: I don't know if this is the correct term, but how would one calculate the time it takes for 3 planets to lie on the same line?


texts:
Where can I find free/open data for the observed (not calculated/theoretical) distance between the Earth & Moon?

I want to see the observed distance of the Moon, hour by hour.


texts: How was the Messenger spacecraft used to measure Mercury's orbital precession to such accuracy? Could this have been done using radar?

texts:
This comment under an answer to Path of Mercury and general relativity mentions that the Messenger spacecraft was used to measure the precession of Mercury's orbit to such accuracy that the tiny component due to the Sun's $J_2$ could be resolved from other components including relativistic effects and the gravitational pull of other solar-system bodies. The value of this component is given there as "about 0.03 arcseconds/century".

That rate corresponds to a motion of the apses of about 500 meters per year, which I roughly estimate translates to an error in Mercury's position that accumulates at about 100 meters per year.

That seems to be within the a range that could be doable with an extended series of radar measurements. For comparison, in the 1960's Venus' diameter was measured to +/- 3000 meters way back in the 1960's and a precision measurement has such importance to general relativity validation.

Questions:

How was the Messenger spacecraft used to measure Mercury's orbital precession to such accuracy?
Why couldn't this have been done earlier using a series of radar observations, or could it have?


texts: How to know when the moon is visible on a particular night?

texts:
I am looking for beginners information on how I can know when the Moon is going to be visible on a particular night ?

Google-fu says to look for moonrise information about my city from websites such as https://www.timeanddate.com/moon/australia/darwin
So if I followed the above guide for today and started walking on a plane field West-NorthWest direction, at 10.01 PM I should be able to see the Moon. Is my understanding correct? Or is there another way to know when Moon will be visible on a particular (clear) night.


texts: How do I calculate the surface temperature of a star from luminosity? I've been trying to delve into speculative solar systems for a bit, and I've heard the surface temperature of a star can be calculated from its brightness, but I can't find any ways to do that! Is it even possible to do so?


texts: What to do after first year of amateur astronomy?

texts:
I have practiced astronomy as a hobby for a while now. I have an entry-level telescope (6-inch Newtonian). In addition to observing the night sky, I have been studying physics related to optics, astronomical distance & temperature measurement, gravity, etc.

My plan is to next get a CCD camera, and do some astrophotography. It should keep me busy the next winter. However, I feel I should try to find some kind of project where I could gradually make progress. Otherwise, after learning all the basics, and seeing everything that can be seen with the smallish telescope I have, I might gradually start to lose interest.

What would you recommend as the "next stage"?


texts: Type II supernovae as a distance indicator

texts: Could someone explain in brief exactly how type II supernovae could be used as distance indicators? as in simply the formulae used and how they are employed? I know there are certain methods based on expansion velocity etc but the open supernova catalog ( https://sne.space/ ) gives the absolute and apparent mag for type II supernovae too, why would we then need to use methods dependent upon expansion velocity and other complications when we can simply calculate distance using $10^{m-M + 5}$?


texts: Is the wind's intensity on Mars similar to Earth?

texts:
I've read that in Mars' poles, the winds can be as fast as 400 km/h, when the poles are exposed to sunlight because the frozen $CO_2$ sublimes. I know that the Martian atmosphere is much thinner than Earth's atmosphere.

So, by knowing the wind speeds on Mars, is there any way to get an idea of its intensity, or in other words, the intensity of a wind of x speed in Mars, to which speed of wind of Earth is comparable, for them to have the same intensity?


texts: If dark matter bends light, how do we know the stuff in the sky is where we think it is?

texts:
We measure movement, position, and many other things of an object in space because of its light and what we can measure with it. But as far as I know there's supposed to be a HUGE amount of dark matter in space whose mass and size we don't know, and because it has mass it has gravity and it can bend light.

I know physics can take into account the gravity of stars and huge stuff in space, but how can they be certain of any of their measurements (specially the position) if they don't know how that light has deviated from a straight line?


texts: Protocol for establishing longitudinal meridians on other heavenly bodies

texts:
When we first observe a new heavenly body (it could be a new star, asteroid, etc.,or a minor planet in our own solar system), are there any procedures set in place for establishing a system of longitudinal meridians?

Being that a prime meridian is an arbitrary concept that you can pick and establish anywhere, is a location decided based on a physical feature, or perhaps from the first points of data we gathered when making detailed observations of the heavenly bodies for the first time?

What about heavenly bodies with no easily discernible or non-stationary features (Gas Giants)? Also does the IAU regulate this process?

I have always wondered if the USSR and the US shared common reference points for locations such as on the moon.


texts: How many stars can be seen naked eye from outside the atmosphere? How dense is the "sky" from there?

texts:
When I was 20, I went to Naxos (Greece) with friends. I was driving my car during the night on the mount Zeus (1003 meters - 3,291 ft) listening Pink Floyd.

Up there we stopped the car to take a walk, looked above and ... wooooooowwwwww!

The sky was dense! Full of stars. We didn't know it could have been like that.

My question is:

How dense is the deep sky from outside the atmosphere?

For example, on the ISS during the night (the Earth that cover the Sun) looking at the deep sky, how many stars can you see? I saw a lot of pictures of the stars taken by the Hubble telescope), but I don't think it can be compared, right?


texts: Problem with Dobsonian reflector telescope

texts:
I'm not sure whether I should ask this question here but I'll give it a shot. I have a XT8 Dobsonian Reflector telescope and it works fine. But recently when I use the focuser to focus the image when I change eyepieces it focuses fine but it makes a scratching noise that is really loud and it doesn't sound right. Now I don't know what this screw that I am about to explain is neccesarly for because I am no where near a telescope expert. Next to the eyepiece there is a screw that you can turn loose and if you make it as tight as possible the focus knob won't focus the image but when it is a little loose it starts to focus it, but it keeps making a scratching sound and a high pitched rubbing sound that doesn't sound right.  I know I explained this pretty vaguly but any help would be helpful! So far I could not find anything on the web for this weird problem.

UPDATE:


texts: How would we detect an Earth doppelganger planet?

texts:
With our current technology, or technology available in the near future (up to 2025), how would we detect a planet exactly like our own, and how close would it have to be to be detectable?

Which methods thus far have proven the most effective for detecting Earth-like planets and what would they be able to reveal regarding our planet's atmosphere, orbital properties, and our species?

This question is interesting food for thought on both how easily an extraterrestrial civilization with comparable technology might find us, and for how easily we might find them.

For the purpose of the question we will assume that there is a planet with a civilisation with equivalent technological advancements that could exist anywhere.


texts: I am only 12 years old and I'm constantly wondering and trying understand how gravity really works. On YouTube everyone always talks about objects warping space time around themselves and uses the analogy of a trampoline. I still don't understand gravity because if space were like a trampoline, then earth would be spiraling in towards the sun along with all the other planets, right? So could someone explain to me how gravity really works without the trampoline analogy?


texts: How big is the Andromeda galaxy in the night sky?

texts:
If divide the distance to the sun by its diameter, I get the number 107.517
Likewise, if divide the distance to the moon by its diameter, I get the number 110.650

Both numbers are quite similar which means both objects look roughly the same size in the sky.

Now, I'll do the same calculation for the Andromeda galaxy.
Distance: 2,537,000 light years
Diameter: 220,000 light years

Quotient: 11.531

I get a number that's roughly 1/10th that of the moon.
This means that Andromeda should look almost 10 times bigger than the moon or the sun.

Is this calculation correct?
Does the Andromeda galaxy really look 10 times bigger than the moon in the night sky?

If it looks so big, why is it so difficult to see with the naked eye?


texts: I created a program to convert the temperature (in Kelvin) of a blackbody to RGB color. However, it is slightly inaccurate, and the deviations increase for values greater than 10000K and less than 1500K. Is there an accurate mathematical function that generates RGB values for given temperature, ranging from ~500K to 20000+ K?


texts: Has celestial navigation been materially impacted by the imperfect nature of celestial reference frames over time?

texts:
In this video on inertial reference frames, it is mentioned that the stars are humanity's best inertial reference frame: the earth experiences a subtle acceleration relative to the sun due to the earth's own orbit and rotation; therefore the earth is only roughly an inertial frame. But, the stars are relatively fixed, they say.

However, the stars in galaxies move relative to each other--I assume many with some sort of rotational velocity and therefore acceleration--and galaxies themselves are all slowly moving away from one another, perhaps even with centrifugal acceleration of their own.

Has this acceleration been great enough that humanity has needed to update navigational tools that rely on the assumption that stars are an inertial reference frame?

Is it possible that every observable object in the universe is in fact accelerating relative to an unobservable reference frame?


texts: Gravitational pull needed to keep a gas in atmosphere

texts:
How can you determine the gravitational force needed to keep a particular gas in the atmosphere of a planet (for example, carbon dioxide (CO2))?

I came across the following formula $\left(\frac{8RT}{πM}\right)^{0.5}$

here: http://www.tau.ac.il/~roichman/CVI/hw1/hw1.pdf

$M$ = Molecular mass

$R$ = Gas constant

$T$ = Temperature

Would this formula apply?


texts: Why are gas giants colored the way they are?

texts:
As I understand it, Jupiter, Saturn, Uranus, and Neptune are all made primarily from varying proportions of hydrogen and helium.  Despite this, Jupiter is very red, Saturn is yellow, and Uranus and Neptune are blue.  Why the difference in colors for planets with similar chemical makeups?  Or do they not have similar chemical makeups?

If such an explanation is available, I would like specific descriptions of why each planet is colored the way it is.


texts:
I'm in the somewhat awkward position of asking something that, despite me thinking there should be hundreds and hundreds of web pages and datasets showing this particular value, I've been unable to find anything about it.

What, indeed, are the Right Ascension and Declination angles of the Sun for J2000 ? Wikipedia shows some values (RA = 286.13, Dec = 63.87), but it is not clear whether they are J2000 or not...


texts: Are electrons and positrons equal in cosmic rays?

texts:
I think the question here is not answered directly. According to Wikipedia,

  Of primary cosmic rays, which originate outside of Earth's atmosphere, about 99% are the nuclei of well-known atoms (stripped of their electron shells), and about 1% are solitary electrons (similar to beta particles). Of the nuclei, about 90% are simple protons (i.e., hydrogen nuclei); 9% are alpha particles, identical to helium nuclei; and 1% are the nuclei of heavier elements, called HZE ions

Together, electrons and positrons need to be neutral in charge, right?
Only 1% are electrons.

How can they be neutral in charge?


texts: How did Astronomers mostly(?) agree to publish arXiv preprints along with peer-reviewed Journals? Was there pushback?

texts:
This excellent, thorough and well-sourced answer to Has a gravitational microlensing event ever been predicted? If so, has it been observed? includes four links to papers on adsabs.harvard.edu pages, and all of these have arXiv preprints.
In fact a large number of posts link to the arXiv preprint version of astronomical papers and I suspect most astronomy papers have arXiv preprints.
To my understanding, this allows other astronomers earlier access to and awareness of ongoing research results which may allow the field to progress faster, and also allows everyone access to the paper without penetrating a paywall. This may have implications for the folks who collect the money which is paid at the wall.
Question: How did astronomers manage generally agree to do this? Was there much pushback by astronomical journals and/or by some authors in the astronomical community? If so, how was that resolved?
note: This may also be on-topic in Academia SE, but I think that both the pools of potential answerers and of interested readers of this bit of history in the field of Astronomy here is substantial.

Obligatory XKCD, hat tip to @JamesK


texts:
Is this picture of a digital camera being oversaturated by bright light source i.e. the Sun?

http://farm4.staticflickr.com/3110/3147227521_396e16bcfc.jpg


texts: What explains this puzzling radio signature from a fireball?

texts:
September 21, 2017 a bright fireball over Holland was reported by hundreds of people. Several all-sky cameras recorded it as well as a special radio beacon set up to capture radio signatures of meteors - the BRAMS beacon at Dourbes, Belgium.

I'm puzzled by what I see in the radio spectrogram from one of the BRAMS stations at Ophain, Belgium :

The orange blob in the center is the radio signature of this fireball.
The horizontal line is the beacon signal (0 Hz), time runs horizontally, frequency runs vertically, positive above and negative below the beacon line.
What is displayed are Doppler shifts.

The track for this fireball has been computed since and relative to the radio receiver at Ophain and radio beacon at Dourbes it is receding from both.
However, the Doppler blob is above the beacon line which is only possible when the fireball recedes from Ophain but approaches the beacon, approaches both, or approaches Ophain but recedes from the beacon - NOT when it recedes from both!

The only way I can make sense out of this is to assume that the ionized
gas trail from the fireball reflecting the radiowaves was NOT receding from the receiver and transmitter beacon - only the fireball was?!

Or are there any other explanations?


texts:
I am trying to plot superwasp lightcurves in python from the fits files, using TMID and FLUX2, getting this:

There is a way to transform the information so I can plot the lightcurves with the kepler format, a example from kepler is something like this:

Any help is useful.


texts:
I'm studying a particular case of the restricted three-body problem. It's been found that some objects follow a horseshoe orbit pattern, and I'm trying to sort out something through an integration code in C. I'm following some advice in the article Families of periodic horseshoe orbits in the restricted three-body problem, which gives me ideal initial conditions and the equations in the centre of mass system. (m is the mass of Earth, and consequent position of the sun in the center of mass reference system, (x,y) are the coordinates of the third body, assumed massless (as the restricted problem requires).

$$ O=(x^2+y^2)/2+\dfrac{(1-m)}{r1}+\dfrac{m}{r2}+\dfrac{(1-m)m}{2} $$
$$r1^2=(x-m)^2+y^2$$
$$r2^2=(x-m+1)^2+y^2$$
$$ a(x)=\dfrac{\mathrm{d}{O}}{\mathrm{d}x} + 2v(y)$$
$$a(y)=\dfrac{\mathrm{d}{O}}{\mathrm{d}y}-2v(x)$$

The positions of the "sun" and "earth" are fixed to (m,0) and (m-1,0), in the same reference system. (rotating reference system, assuming the earth has a circular orbit.)

From all this I've calculated the equations to describe the system:

$$a(x)=x+\dfrac{(m-1)(x-m)}{((x-m)^2+y^2)^1.5}-\dfrac{2m(x-m+1)}{((x-m+1)^2+y^2)^1.5} + 2v(y)$$
$$a(y)=y-\dfrac{y(1-m)}{((x-m)^2+y^2)^1.5} -\dfrac{2ym}{((x-m+1)^2+y^2)^1.5} -2v(x)$$

I've used the algorithm of Runge-Kutta 4 to integrate those equations.
(I know the code is quite mind-twisting, but I just can't use pointers and I use structs everywhere).

#include<stdio.h>
#include<stdlib.h>
#include<math.h>

#define dt 0.0001
#define N 100

typedef struct{
    long double x,y;
}vec;

typedef struct{
    vec k1,k2,k3,k4;
}runge;

typedef struct{
    runge r,v;
}big;

double dS,dE,m;

double accx(double,double,double);
double accy(double,double,double);
void rad(vec);
big rungekutta(vec,vec);
vec moto(vec,runge);
double jacobi(vec);

int main(){
  vec r,v;
    big f;
    double J,t;
    int i,Num;
    FILE* s1;
    s1=fopen("HorseShoe.dat","w");

    Num=(int)N/dt;
    scanf("%Lf",&r.x);
    scanf("%Lf",&r.y);
    scanf("%Lf",&v.x);
    scanf("%Lf",&v.y);
    scanf("%lf",&m);

    for(i=0;i<Num;i++){
        t=(i+1)*dt;
        rad(r);
        f=rungekutta(r,v);
        r=moto(r,f.r);
        v=moto(v,f.v);
        J=jacobi(r);
        fprintf(s1,"%lf\t%Lf\t%Lf\t%Lf\t%Lf\t%lf\n",t,r.x,r.y,v.x,v.y,J);
    }
return 0;
}

void rad(vec r){
    dS=pow(r.x-m,2)+pow(r.y,2);
    dE=pow(r.x-m+1,2)+pow(r.y,2);
}

double jacobi(vec r){
    return pow(r.x,2)+pow(r.y,2)+2*(1-m)/dS+2*m/dE+m*(1-m);
}

double accx(double x,double y,double v){
    return x-(x-m)*(1-m)/pow(pow(x-m,2)+pow(y,2),1.5)-m*(x-m+1)/pow(pow(x-m+1,2)+pow(y,2),1.5)+2*v;
}

double accy(double x,double y,double v){
    return y-(1-m)*y/pow(pow(y,2)+pow(x-m,2),1.5)-m*y/pow(pow(y,2)+pow(x-m+1,2),1.5)-2*v;
}

big rungekutta(vec r,vec v){
    big f;
    f.r.k1.x=v.x;
    f.r.k1.y=v.y;
    f.v.k1.x=accx(r.x,r.y,v.y);
    f.v.k1.y=accy(r.x,r.y,v.x);
    f.r.k2.x=v.x+f.v.k1.x*dt/2;
    f.r.k2.y=v.y+f.v.k1.y*dt/2;
    f.v.k2.x=accx(r.x+f.r.k1.x*dt/2,r.y+f.r.k1.y*dt/2,v.y+f.v.k1.y*dt/2);
    f.v.k2.y=accy(r.x+f.r.k1.x*dt/2,r.y+f.r.k1.y*dt/2,v.x+f.v.k1.x*dt/2);
    f.r.k3.x=v.x+f.v.k2.x*dt/2;
    f.r.k3.y=v.y+f.v.k2.y*dt/2;
    f.v.k3.x=accx(r.x+f.r.k2.x*dt/2,r.y+f.r.k2.y*dt/2,v.y+f.v.k2.y*dt/2);
    f.v.k3.y=accy(r.x+f.r.k2.x*dt/2,r.y+f.r.k2.y*dt/2,v.x+f.v.k2.x*dt/2);
    f.r.k4.x=v.x+f.v.k3.x*dt;
    f.r.k4.y=v.y+f.v.k3.y*dt;
    f.v.k4.x=accx(r.x+f.r.k3.x*dt,r.y+f.r.k3.y*dt,v.y+f.v.k3.y*dt);
    f.v.k4.y=accy(r.x+f.r.k3.x*dt,r.y+f.r.k3.y*dt,v.x+f.v.k3.x*dt);
    return f;
}

vec moto(vec r,runge rk){
    r.x+=(rk.k1.x+2*rk.k2.x+2*rk.k3.x+rk.k4.x)*dt/6;
    r.y+=(rk.k1.y+2*rk.k2.y+2*rk.k3.y+rk.k4.y)*dt/6;
    return r;
}

Plotting the results I just get a spiral, while using the given inputs I should get a horseshoe orbit. I've tried many different inputs, (m=0.0001 and m=0.000003, the latter is in scale with the actual values of Earth and Sun masses (sun mass is 1-m)).

I just can't see what's wrong (probably everything :D), can anyone please help me?


texts: Are galaxies expanding away from us at more than the speed of light?

texts:
If you go to the following video, at this certain time:

https://youtu.be/Iy7NzjCmUf0?t=435

you will see that the narrator says:

  "We simply don't know what else is out there because the light from these incredibly distant places has not yet had enough time in the universes history to reach us yet back on Earth. And the light from some places might never reach us at all. Because some parts of space, very far from Earth, are expanding away from us, faster then the speed of light, that means that the light from these places will never, in an infinite amount of time reach Earth"

Am I understanding this correctly? Some parts in the universe are expanding faster then the speed of light? Is it yet proven that there are definitely things faster then the speed of light?

I feel like I am misunderstanding the above statement.

P.S, try watching the entire video, it will be 10 minutes well spent.


texts: Do we finally have a decent set of parameters of 162173 Ryugu?

texts:
"Give a man a watch and he will know the time; give him two and he will never be sure" - this was my impression when reading the Wikipedia article on 162173 Ryugu.

Mean diameter:

0.865±0.015 km[3]
0.87 km[4]
0.90±0.14 km[5]
0.92±0.12 km[6]
0.980±0.029 km[7]
1.13±0.03 km[8]

And no mass estimates at all.
Now that Hayabusa2 has arrived, do we have solid measurements of Ryugu's size and mass yet?


texts: rotation curve in the innermost part of galaxies

texts: I read in Sofue's articles (1999) that he used a method to investigate the bulge of galaxies to obtain rotation curve of this part of galaxies. So my question is how could Vera Rubin et al. obtained the rotation curve in their article (1978), especially the steep increase in the begenning of the curve since in Sofue's article he seems to say it was rather difficult, at that time, to investigate galactic bulges ?


texts: Can a star be massive enough to go 'supernova' yet not be massive enough to leave behind a neutron star? Or, perhaps vice versa?

texts:
From what I understand, stars that explode as a supernova at the end of their lives become either a black hole or a neutron star, and less massive stars that do not explode become white dwarves, but...
Do those two mass limits overlap exactly?


texts: How is it possible that the CMB approaches the earth from all directions?

texts:
I presume that the photons from the CMB approach the earth from all directions, otherwise we couldn't detect them with a picture where it is present everywhere in the universe with a tiny anisotropy.

Now the CMB is a result of the big bang. Another result of the big bang is that in all directions we see galaxies moving away from us, like we are in the center of the universe (which is probably not the case).

But why are all the galaxies (except a few) moving away from us while the CMB is approching us. What is the reason for that difference? Or aren't those photons coming from everywhere?


texts:
Is there a reason that the Earth has the tilt that it does (~23°)?
How do we know which way is supposed to be 0°?
Does this tilt have major consequences on the planet?
Has it changed and will it change in the future? (If so, would there be any consequences?)


texts: Is it proper to refer to objects beyond Neptune as "Kuiper Belt Objects?"

texts: It seems to me that nowadays the more proper term is "Trans-Neptunian Object".  Is there any difference between Kuiper Belt Objects and Trans-Neptunian Objects?  If not, is one term preferred over the other?


texts: How many supernovae events are on their way from within the Milky Way?

texts:
The Milky Way has a diameter of around 100.000 light years with the Solar system placed some 25,000 light years from the center.
If we assume a supernova rate of 1 per century, and also assume that supernovae are distributed somewhat uniformly in our galaxy, then it seems to me that light and neutrinos from hundreds maybe even thousands of these events are propagating through the Milky Way towards us at this very moment.
Can this be estimated more accurately? Does the supernovae distribution even affect this estimate, given the age of the galaxy?


texts: Polynesian wayfinding techniques - how does this work?

texts:
In Disney's "Moana", we get a hint of a Polynesian wayfinding technique of holding your hand to "measure the stars" (note, you're not giving the sky a high-five). We get to see two different constellations measured: Crux (the Southern Cross), and Orion.

By matching the positions of the stars either in real life or on Stellarium, I was able to figure out something of how the Southern Cross is used. You place your fingers against the four brightest stars (two pointers plus the two brightest of the cross itself), and then the south celestial pole is right in the curve of your outstretched thumb. (Aside: It makes an awesome demo in Stellarium to place your hand on the screen, do the measurements, and then turn on the grid, highlighting the south pole beautifully.)

What I haven't managed to do, though, is match the corresponding measurement in Orion. The fingers are placed near Orion's belt and heel, but I can't figure out what is being measured. Can anyone provide either certain information, or plausible guesses?

(I can provide a screencap if there's an appropriate way to include it in the question.)


texts: What is the current routine of modern astronomy?

texts:
With about all visible stars catalogued, measured and photographed it seems that chapter is closed. I realize currently while amateur astronomy is big on cataloguing all asteroids in the Solar System, the "Big Boys" are now primarily busy with discovery of extrasolar planets. But that's certainly not all. Sure there's a lot of unique research, new, unexpected discoveries, assorted "odd" research like SETI, and so on, but I'd like to know what is the "daily bread" of every astronomer nowadays?

What are the current typical, routine tasks of an average professional astronomer? What is being observed/measured/searched for? What challenges occupy most effort and time? Or am I wrong assuming there even is such a thing, and every observatory is a unique snowflake?


texts: Observed instances of massive gas inflow and AGN restart due to cluster-cluster merger?

texts:
Works such as Acreman et al. (2003) have demonstrated via simulations the possibility of gas being stripped off a galaxy due to interaction with a galaxy cluster.
Have there been confirmed instances where a cluster-cluster merger led to the infall of huge amounts of gas onto one of the clusters? Further, have there been instances of cluster-cluster merger leading to restarting Active Galactic Nuclei (AGN) activity?


texts: How to calculate the orbits of Neptune and Pluto based on the orbits of Uranus and Neptune respectively?

texts:
I get the below from Neptune and Pluto.

In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's neighbour Uranus.[22] Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesise that an unknown body was perturbing the orbit through gravitational interaction.[23] In 1843, John Couch Adams began work on the orbit of Uranus using the data he had. Via Cambridge Observatory director James Challis, he requested extra data from Sir George Airy, the Astronomer Royal, who supplied it in February 1844. Adams continued to work in 1845–46 and produced several different estimates of a new planet.
In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus.[26] Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune.

Does anyone know how did people calculate their orbits in 19th century? It is not a three-body problem, right?


texts: How did the lighter elements end up in the center of the solar system? Solar System Formation

texts: The previous generation of stars famously are the origin of all the heavier elements (up until iron?) in the solar system. So a big portion of the solar system mass actually is made up of Carbon, Silicon, Iron and the like because of that. But in the center, and only in the center, there is a star with presumably almost no heavy elements inside. How can that be? Am I wrong about the actual mass concentrations or is there really an imbalance, i.e. is the element distribution really lighter towards the center of the solar system? I would presume that the previous generation of stars just ended in a more or less uniform cloud of debris, from which the solar system formed. But if so, why aren’t there star systems where the star has a very different composition and is kind of a spluttering, dirty fusion machine (metaphorically, I mean)?


texts: I'm just curious about that the existence of non-rotating stars. Is it physically possible for a star not to rotate at all? Does magnetic braking eventually stop the stellar rotation?


texts: I searched Google for a while and of so many available cameras, I couldn't find a moon live stream video camera. Does anyone know any free website that offers this? Where can you see live the moon right now?


texts: Methods for testing a refracting telescope lens

texts:
I have an old (nearly 50 years old)alt-azimuth three-inch (7.6cm) refracting telescope that someone else made. They ground the lens, created the cylindrical body and added a rack and pinion style focusing slider. It is mounted on a homemade tripod.

This has been poorly stored for several years in damp conditions and has developed mould on the main lens elements. The main lens elements comprise a convex and concave combination (presumably to reduce achromatic aberration?)

Question:

I've cleaned the main lens elements in soapy water and removed the mould using a soft cloth.

How would you now test this lens?
Would you reassemble the telescope and check resolution against known double stars or are their tests I can apply to the lens before reassembling the telescope?


texts: As matter approaches a black hole, does it speed up?

texts: If so, how do we know it speeds up?  Doesn't time slow down as gravity increases?  If time slows down around a black hole, is it possible matter doesn't actually speed up?


texts: How large must an object be to be seen through a telescope?

texts: I've studied this in a few optics classes before, but I'm drawing a blank at the moment.  If I know the aperture size and focal length of a telescope, as well as how far away the object I want to view is, can I calculate how large/wide/big that object must be in order to be seen?


texts: What would happen when the speed of the Sun's spin changes?

texts: Does the Sun's spin have effect on the orbit of the Earth?


texts: Converting ångström spectral dimension to galaxy speed (km/s)

texts:
I have a spectral cube (in FITS format) whose spectral dimension is in ångströms. The sampling along the spectral dimension is 0.28A (CDELT=0.28). The observation in the cube is Ha emission of a galaxy at redshift 0.1.

How can I convert the sampling to km/s?


texts: How would I search within an angular distance of an object corresponding to a distance in parsecs?

texts: I'm interested in the learning about the local densities around galaxies. I've found myself a bit confused on how to relate angular distances (arcseconds) with physical distances (parsecs) when thinking about distances between two distant galaxies. For example, let's say the average distance between galaxies within clusters is a few megaparsecs. If I had ra, dec coordinates of a particular galaxy and could hypothetically query an all-sky catalog, let's say I'd want to search within a few megaparsecs to get a feel for its density field. But generally when querying catalogs you specify a search radius in some arcseconds. How would these relate here?


texts: How long does it typically take to make a telescope yourself for the first time?

texts: How difficult is it to make a telescope oneself? How much time would it take for a first-timer? What are the typical pitfalls?


texts: At what density does helium burning start in a star?

texts: I have seen several references that say that helium burning begins in a star once the core temperature reaches $10^8$K (such as here) but nowhere that says what density that corresponds to.  Does anybody know a reference that has this value?  And if it makes a difference, I would like to know the density for quiescent helium burning, not helium flash helium burning.


texts: What do the words "p-type" and "s-type" mean?

texts:
As explained in various sources (such as that answer), a planet in an s-type orbit orbits one of the stars of a binary star system, whereas a planet in a p-type orbit orbits both stars. A synonym for a p-type orbit (at least in binary star systems, not sure about N-ary systems in general) is "circumbinary", which is a self-explanatory term.
However, what do "s-type" and "p-type" mean, respectively?
Are "S" and "P" acronyms of anything?


texts:
Whenever you take a look at a photo that is taken by hubble,
Lets say M87 or Eagle Nebula or maybe Ultra Deep space field,

The Image is always "Cut"

Like this :

or this

I want to know reason for this ,

Any ideas?

I think it's during photo capture or processing error... or no details about elements to color?


texts: Why planets and stars are always round/oval in size?

texts: What kind of stability it provides to each and every celestial body that each one them are round in shape?


texts:
Firstly, I have to admit that I don't fully comprehend the concept of "CMB Polarization". When I think of "Polarization", I normally think of the optical definition of polarization, as defined by the electric field. What I currently think is that CMB Polarization is simply the temperature gradient of the CMB. The Wikipedia page also said that:

  The cosmic microwave background is polarized at the level of a few microkelvin.

Since microkelvin is a unit of temperature, CMB Polarization must be somehow related to temperature. Is CMB Polarization simply the temperature gradient of the CMB?


texts: Could some stars be orbiting galaxies in outward spirals instead of perfectly round circulation? Patterns of matter movement relate strongly to the galaxy centre. A special relationship between the galaxy and the central object suggest the centre may be an origin point in 'outflow'. The BH central object behaves strangely by instigating the release outwardly of massive amounts of matter. This may form stars that spiral out across the galaxy. The grand design of features in galaxies may related to general outward spiralling.


texts: If a large satellite keeps a planet from wobbling,why is Pluto/Charon on its side?

texts:
I understand that a “large” moon, like ours, keeps the planet’s axis from being peturbed, because the spin axis and the moon’s orbital plane exchange momentum and tend to stay in the same plane, and the orbital angular momentum is much stronger because of the long lever arm. So, tipping the planet means changing the moon’s orbital plane too, and the peturbations don’t make a significant change to its large value.

So, why is Pluto, along with the orbital plane of its satellites, tipped nearly 90 degrees?


texts:
In several papers about Recombination, (for instance Peebles, Astrophysical Journal, vol. 153, eq. 39) it is claimed that the evolution of matter temperature follows the equation
\begin{equation}
\frac{dT}{dt} = -2\frac{\dot{a}}{a}T + \frac{2}{3 K_B n} \epsilon
\end{equation}
where the first term in the right hand side accounts for the expansion of the universe and $\epsilon$ is the energy injected into the gas per second per unit physical volume. How can I derive this equation?
I suppose that this equation can be derived applying the First Principle of Thermodynamics to a comoving volume in the expanding universe or maybe taking the second moment of a Boltzmann equation. Any help or reference in which this equation is discussed would be very helpful.


texts: Can a tidally locked planet have their own habitable zone?

texts: As we know, if a planet is near a star and is tidally locked, then at near side it is very hot and at far side it is very cold. But between 2 sides there should be a gradient of temperature change and should have an area which the temperature is suitable for lives. Can we also say it has a habitable zone?


texts: Wouldn't be "dark matter" better called "massless gravity"?

texts: In spite of being a simple words game, that would make me feel less uncomfortable, as there could be a mysterious field still less mysterious than matter that passes through itself etc. Moreover, couldn't be that there exist some high order terms that play a role only when mass reach big values, i. e. those comparable to a galactic mass? How/why this cannot be the case in general relativity?


texts: Do the Bullet Cluster remnants prove that dark matter consists out of particles?

texts:
Some time ago the remnants of a collision between two collections of stars were discovered. These remnants are called the Bullet Cluster:

The color blue (non-visible in reality) represents the presence of dark matter. Pink represents the normal matter (I'm not sure if the color pink in this picture is actually visible, contrary to the blue).
There are some theories around to explain the nature of dark matter. They can roughly be divided into two groups:
1)Theories that state, dark matter consists of particles
2)Theories that modify gravity (apart from MOND) itself, for example, the theory propagated by the Dutch physicist Erik Verlinde (who's "contraption" here in the Netherlands was announced on TV as a huge breakthrough and for he received the Spinoza Prize. i.e. a lot of money, in 2011; much too much, in my opinion) which see gravity as entropic.
Theories of a modified gravity make the prediction that dark matter is tied to normal matter. Now in pictures of the bullet cluster, it seems like matter and dark matter are clearly separated.
Does this prove that dark matter has to consist out of particles?


texts: How to identify component of velocity of a star from its red shift?

texts:
When a telescope observes a distant star, all it sees is the light being emitted by the star. Observing the wavelength of this light, we determine the velocity of the star. If the wavelength is red-shifted (longer) then the star is moving away from us, and if the wavelength is blue-shifted (shorter) then the star is moving towards us.

But how do we know that the wavelength that we see is not the red shifted wavelength but the original wavelength emitted by the star? Do we already know what wavelength of light the star is supposed to emit (if yes, how?)

Also, using this method we should be able to identify only the radial component of velocity. Because the tangential component of velocity of the star would have no effect on the wavelength that we observe? So how do we determine the actual velocity (including the proper direction) of the star?


texts: If we found evidence of life on Mars, how would we know that it originated on Mars rather than Earth?

texts:
Consider this scenario.  One of the probes sent to Mars either very soon, or within the next century, finds conclusive micro-fossil evidence that life once existed on Mars.

One theory today that is quite common is that life might that have arisen on Mars was then ejected into space, floated to Earth and then seeded life on our planet.

I'm tracking with you and that could make sense, assuming we found the conclusive evidence for life on Mars.  However, why would we assume that the early martian life originated on Mars, rather than Earth?  Is it not likely that somewhere between 2.5-3 billion years ago, a major volcanic eruption or meteor impact could have hit an earth teeming with simple-life and sent it hurdling through space to Mars?

I understand that my theory has the gravity of the sun working against it, but I also see it not being entirely impossible.  I mean, 100 years ago, one could assume that life originating on Mars and then being sent this was was ridiculous and impossible but now it's a rational, viable theory.

Considering such a scenario, how would scientist verify that these life-forms didn't originate from Earth, rather than Mars?  It seems to me that this question is the first question that would need to be asked and answered after the fossil-records were found and verified.


texts: Do we see planet Venus every day? (Just before sun sets or Just after sun sets) throughout year? or some days we don't see it?


texts: Can anyone help me by defining what is exactly Bondi Radius? I have searched far and wide but the results are very complicated and so far I have only rough idea of it but I just cant explain it very well


texts: Is it possible to create a traditional clock solar system?

texts:
Is it possible to create the following system of satellites?:

Place a satellite that orbits the some object X, once per day. Call it D.
Place a satellite that orbits D, once per hour. Call it H
Place a satellite that orbits H, once per minute. Call it M.
Place a satellite that orbits M, once per second. Call it S.

If possible, what are some values for the weights, rotational speed, and radii that would work?

Also, does the universe have a limit on how many layers of satellite systems it permits?


texts: What is the connection between moons, rings, and gaps?

texts: I was watching this video and they said that the rock probably has a moon. Is the reason they assume it has a moon is because there is a gap between the rings created by the moon's orbit around the rock? Or the rings created by the moon breaking down in its orbit and leaving debris behind in its orbit?


texts: Does any asteroid we have ever seen have a hot core like earth.?

texts:
I was wondering if any of the asteroids we have ever seen has a hot core like Earth's.

I'm thinking that Earth has a strong gravitational field and so formation would have involved a lot of gravitational energy being converted to heat, but not so much for the asteroids.


texts: What exactly is a "conclusive association" in Astronomy?

texts:
Phys.org's Astronomers use slime mold model to reveal dark threads of the cosmic web says

A computational approach inspired by the growth patterns of a bright yellow slime mold has enabled a team of astronomers and computer scientists at UC Santa Cruz to trace the filaments of the cosmic web that connects galaxies throughout the universe.
Their results, published March 10 in Astrophysical Journal Letters, provide the first conclusive association between the diffuse gas in the space between galaxies and the large-scale structure of the cosmic web predicted by cosmological theory.

This links to Joseph N. Burchett et al, Revealing the Dark Threads of the Cosmic Web, The Astrophysical Journal (2020)
Kudos for the use of the slime mold model, this organism is a gift that keeps on giving to not only biologists but a wide range of other fields https://www.google.com/search?q=slime+mold+growth+modeling which includes computing, network analysis, transportation and even cosmology.
Astronomy differs from some other fields of science in that most hypotheses must be tested by observation of naturally occurring phenomena, it's hard to test them through carefully designed experiments. While particle physicists can "throw large hadrons" at each other under controlled conditions, astronomers can't similarly "collide white dwarfs" in the laboratory.
Question: In the results described in the ApJ Letter, was there such a thing as a "conclusive association"? Is that even a thing? Or was there simply a similarity as defined by some metric which the authors believe to be reasonably objective?


texts: How can the orbit of a Kuiper Belt Object be differentiated from the transit of a rogue planet?

texts: Related to the question "Are any Pluto-sized objects remaining to be discovered in the Kuiper Belt?" and the fact that most of the Kuiper Belt objects have very elliptical orbits, the question begs, what observational techniques are used to confirm that the object being viewed is an orbiting Kuiper Belt Object, as opposed to the transit of a rogue or 'orphan' planet?


texts: Determining effect of small variable force on planetary perihelion precession

texts:
Is there an analytical technique for determining the effect of a small variable transverse acceleration upon the rate of aspides precession (strictly not a precession but rotation of the line of aspides) of a planet orbitting around the Sun in a 2D plane according to Newtonian gravity law?

I have modelled such effects in a reiterative computer model and would like to verify those measurements.

The transverse acceleration formula is

  $$At = (K/c^2)*Vr*Vt * Ar.$$

Where:-

  c is speed of light,

  K is a constant of magnitude between 0 and +/-3, such that $K/(c^2) << 1$.

  Ar is the acceleration of the planet towards the Sun due to Newtonian gravitational influence of the Sun, ($Ar = GM/r^2$).

  Vr is radial component of planet velocity relative to the Sun (+ = motion away from the Sun)

  Vt is transverse  component of planet velocity relative to the Sun (+ = direction of planet forward motion along its orbital path). Vectorially Vt = V - Vr where V is the total instantaneous velocity vector of the planet relative to the Sun.

  Assume planet mass is small relative to the Sun

  No other bodies are in the system

  All motions and accelerations are confined to the two-dimensional plane of the orbit.

UPDATE

The reason why this is interesting to me is that a value of K = +3 in my computer model produces anomalous (Non-Newtonian) periapse rotation rate values very close within about 1% of those predicted by General Relativity and within a few percent of those observed by astronomers (Le Verrier, updated by Newcomb).

Formula (Einstein, 1915) for GR-derived periapse rotation (radians per orbit)
from http://en.wikipedia.org/wiki/Apsidal_precession
$$
\boldsymbol{\omega}=24.\pi^3.a^2.T^{-2}.c^{-2}.(1-e^2)^{-1}
$$

UPDATE 4

I have accepted Walter's answer.  Not only did he answer the original question (Is there a technique...?) but also his analysis produces a formula which not only confirms the computer-simulated effects of the transverse acceleration formula (for K = 3) but which also (unexpectedly to me) is essentially equivalent to the Einstein 1915 formula.

from Walter's Summary (in Walter's answer below):-

: (from first order peturbation analysis) semi-major axis and eccentricity are unchanged, but the direction of periapse rotates in the plane of the orbit at rate $$ \omega=\Omega \frac{v_c^2}{c^2} \frac{K}{1-e^2}, $$ where $\Omega$ is the orbital frequency and $v_c=\Omega a$ with $a$ the semi-major axis. Note that (for $K=3$) this agrees with the general relativity (GR) precession rate at order $v_c^2/c^2$ (given by Einstein 1915).


texts: Formation mechanism of a non-barred 6-arm spiral galaxy

texts: UGC 12588 has an unusual morphology of an old dwarf spheroidal with six very young spiral arms. None of the 24 Simbad references describe its morphology or recent history from the galactic assembly perspective. It lies on the outer edge of the Local Sheet well outside the virial radius of likely interaction candidates. There are several mysteries about it, but the apparent six-arm morphology (if real) is hard to fathom. The HLA proposal was made in Jan 2020 by Brent Tully, but his more recent papers on ADS do not analyse this particular galaxy. The HESTIA project described by Libeskind 2020 simulates surface brightness profiles of dSph galaxies but does not address the tidal and shear stresses that might explain UGC's recent star-forming episode or that 6-arm structure. I'm not keen on guesswork, but that's where I have arrived. Thoughts?


texts: Has anyone back-calculated previous close encounters between the Apophis asteroid and Earth?

texts:
The Apophis asteroid was only discovered in 2004, and shares an orbit similar to Earth's orbit, leading to many close encounters on the scale of decades.
Most of the stories about this asteroid are about predicting future encounters between Earth and Apophis.  It occurs to me that you can use the same models that predict future encounters to show past encounters as well.
Has anyone done this, and shown any past events when the asteroid has come extremely close to the Earth?


texts: Why Helium could be produced in first minutes of big bang but not heavier elements

texts: At what extent could be sure that no heavier elements than helium could be produced, if we know that matter in universe today is about 25% composed on helium, why could be so sure that the kind of reaction He-He-He diden't took place?


texts: Why do particles from a comet that result in meteor showers spread out mostly along the comet's orbit?

texts:
My understanding of the meteor showers that repeat yearly is that the Earth passes through the orbit of a comet that is nowhere near us, and might not even exist anymore, yet it's orbit is filled with a stream of dust in the same heliocentric orbit by way ahead of or behind their cometary source by tens of millions of kilometers or more.

How do these particles spread out along the comet's orbit so far and yet remain tightly correlated transverse to the orbit? Some meteor showers like the "Unicorn shower" linked below can be quite short, lasting only a fraction of a day!

What makes some meteor showers continue for days, while the "Unicorn shower" can be shorter than one hour?
Meteor shower predicted to be best viewed BEFORE midnight (for reasons other than the moon)?
Why are these objects moving at Vastly Different Speeds along the same orbit?

Left: From What (actually) is Jupiter doing to this year's Perseids meteor shower? Right: From Why would the Perseids meteor rate fall off after maximum faster than the increase before maximum? - click each for full size


texts: Would magnetic reversals on Ganymede affect radiation levels at the surface?

texts: Currently the intrinsic magnetic field of Ganymede is anti-aligned with respect to the Jovian magnetic field. If this situation changed to become aligned, either due to magnetic reversals of the Ganymedean dynamo or due to the reversal of Jupiter's magnetic field (which has been predicted to undergo solar-like reversals on timescales of the order of centuries), would there be any changes to the radiation levels at the surface?


texts: What is the distance of the Earth's orbit around the Moon?

texts: I realize the Earth and the Moon both orbit around their shared center of mass, and that in the case of the Earth and Moon this center is "inside" the Earth. However, I'm looking for an approach to determine the distance the Earth travels in one "orbit around the moon" (as small as it may be).


texts: Where is Voyager 1 (or 2) going? In which direction?

texts: Knowing it has exited the solar system, is it traveling in the direction of the center of the galaxy, away from it, orbiting it, or maybe even going perpendicular from the disk-shaped galaxy?


texts: What consequences does a positive muon anomalous magnetic moment have for astronomy?

texts:
On April 7th, 2021, the muon $(g-2)$ collaboration published Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm, a result which made it to standard news, partly under headlines like Farewell to the Standard Model (German source: Abschied vom Standardmodell)
The following quote from phys.org nicely summarizes what it is in general about:

The experiment at Brookhaven indicated that g-2 differed from the theoretical prediction by a few parts per million. This miniscule difference hinted at the existence of unknown interactions between the muon and the magnetic field—interactions that could involve new particles or forces.

I am now wondering if there are theoretical studies which illuminate what concrete effects a $g-2 > 0$ would have on astronomy, as e.g. Dominik Stöckinger states (translation by me):

Some variants of supersymmetry, in which so-called superpartner particles also explain dark matter, are no longer tenable by this measurement.

Related

An answer to What is the closest candidate for Dark Matter currently? points out that WIMPS are currently the most appealing candidate for dark matter.


texts: Has the rate of neutrino production in the universe varied over time? Are there more or less being created now against the early days?


texts:
This is problem 1-4 from Principles of Stellar Evolution and Nucleosynthesis by Clayton:

Assuming at the Earth a characteristic velocity of 400km/s and density of 10amu/cm$^{3}$ for the solar wind, calculate the rate of mass loss for the sun.

There weren't any formulas about this in the section so I made a stab at it with dimensional analysis.

$$\frac{dM}{dt} = \frac{\rho V}{\Delta t} = \rho v A$$
$$\frac{dM}{dt} = \left( \frac{10 amu}{cm^{3}} \right) \left( \frac{400km}{s} \right) \left( \frac{4 \pi (6.96e10 cm)^{2}}{1} \right) \left( \frac{10^{5}cm}{km} \right) \left(\frac{10^{-24} g}{1 amu} \right) \left( \frac{M_{\odot}}{2 \times 10^{33} g} \right) \left( \frac{3600s}{hr} \right) \left( \frac{24 hr}{day}\right)\left(\frac{365day}{yr}\right)$$
$$\frac{dM}{dt} = 3.84 \times 10^{-19} M_{\odot} / yr $$

However, the answer given in the book is $0.4 \times 10^{-13} M_{\odot} / yr$. So, I'm off by about five magnitudes. Can anyone point out where I went wrong and/or point me in the correct direction?


texts: Can Earth host another natural satellite than the Moon?

texts:
Are there orbits with long-term stability (astronomical time scales) around Earth? Low altitude orbits will decay due to friction with Earth's atmosphere. Inside Earth's Roche limit, a Moon could not form anyway due tidal forces. At altitudes closer to that of the Moon, the Moon would presumably destabilize an orbit such that the object would either collide with Earth or the Moon or be ejected.

Many of the other planets in our solar system have multiple natural satellites; where this occurs, they are always vastly smaller than the primary - think of Jupiter, Saturn, and even Mars. On the other hand Earth, and apparently Pluto, each has only one natural satellite, relatively large in comparison to the primary. Is this because a large satellite will tend to clear the entire "sphere of influence" around its primary of any other material?

So my question is: does the existence of our Moon effectively prevent the formation or capture of any other natural satellite around Earth? (Considering only true satellites, not "companion" objects or quasi-satellites)


texts:
I am doing a project which requires that I have data for the mass of several stars. However, the data for mass of stars is obviously not readily available as it's really hard to calculate. In such a scenario, would it be alright to use the mass-luminosity relationship with different values of p(3.5 for a main sequence star) without getting major errors?

Also, what value of p should i use for other stars like white and brown dwarfs?


texts: Measure an angle in the sky from a photo?

texts: Let's say I have an picture of the sky or of an landscape and I want to measure an angle in the celestial sphere. Is there any software for that? In real life I would use the empirical hand gestures or a theodolite.


texts:
I know an asteroid can have moons, but how about rings? Does it also get affected by the Roche limit?

The asteroid Florence has two moons.


texts: In zero gravity - I'm floating inside a torus as they spin it up, do I float do the "ground"?

texts:
Imagine this scenario - I'm in a space station with one of those huge torus shaped constructions, that you spin to simulate gravity through centripetal force. But it's turned off right now, so I'm floating in the air.

When happens when they turn it on and it starts spinning?
I can imagine what would happen if I was grabbing on to something. I'd be pulled around, and the angular velocity would drag me to the outer part of the torus. But if I'm just floating there, with no friction to the ground to catch me, do I stay put, with the thing just whizzing past me?


texts: What is the dielectric constant of a star?

texts:
What is the dielectric constant of a star, especially its corona? Is it of order 1 or is it quite large?

How much do "impurities" (elements other than H and He) affect the dielectric constant?


texts: To what extent are structure formations unexplained?

texts:
I hear conflicting claims about our understanding of the structure formation in our universe. I heard that ΛCDM explaines well the large scale structure of the universe i.e.

  Comparison of the model with observations is very successful on large
  scales (larger than galaxies, up to the observable horizon)

and then I read the opposite on nasa.gov:

  However, we still do not understand how these structures condensed
  out of the 'primordial soup'. The formation of structure appears much
  quicker than the current theories would have it!

So what is Lambda-CDM actually good at and what are its potential weaknesses, if there is any?


texts: How do we estimate the dimensions and mass distribution of a galaxy's dark matter halo, beyond the effective limits of its visible matter?

texts:
Current research suggests that the Milky Way's dark matter halo may extend up to about 10 - 15 times the size of the visible galaxy, and is believed to be roughly spherically symmetrical. But I don't understand how we deduce that.
In the case of a spherically symmetrical matter distribution, such as the classic "planet with a hole drilled through it", the matter within the radius of the object acts as a single mass at the centre, and the matter outside that radius makes no net contribution to force or motion. So if we somehow stood in such a shaft drilled through the Earth, and had no idea how far above us the shaft extended, we couldn't deduce the mass of any thin shell of matter at distance D from Earth's centre, because the net contribution would cancel out whatever that mass was. So we couldn't conclude much about Earth's higher-up mass distribution, or where this became effectively zero.  (We also couldn't deduce anything from depth if we couldn't measure ambient pressure, which might help.)
By analogy, I don't understand how we can draw conclusions about the extent of matter outside a star or region's galactic orbit.
So how can we deduce the mass distribution, or effective extent/size, of a galaxy's dark matter halo, in a "thin shell" region of space far from the galaxy's centre, if there are no visible stars of that galaxy to provide evidence of the local force of gravity at distances beyond the visible galactic extent?
My present understanding, for info:
I understand in overview some of the principles about a galaxy's dark matter halo, including why we believe dark matter is present and such a large proportion of a galaxy's mass, the need for extra mass to explain how a galaxy retains cohesion/rotation, and the fact that unlike visible matter, dark matter is limited in how dense it can get (Virial theorem? of which I think I may understand the conclusion and recognise the name, but nothing more?). I also understand that given a star (or galactic region's average) motion, we can deduce how much mass lies within that orbit, but I would naively expect a symmetrical "thin shell" of matter outside that orbit to have zero net contribution? But while this seems applicable, I'm less sure.


texts: How much mass will the Sun have when it becomes a white dwarf?

texts:
In 4 billion years, when our Sun sheds all of its outer gas layers and turns into a white dwarf, how much mass will the white dwarf have compared to what the sun has today?

Will the planets still orbit in the same way, or will the reduced mass cause the planets' trajectories to change, so that they eventually leave the solar system?


texts: Does the cosmological principle apply to the entire universe?

texts: Some cosmological models assume that the universe is isotropic and homogeneous and that is also flat and infinite. If the universe is infinite now it was infinite immediately after the big bang. If universe was infinite immediately after the big bang, what explains isotropy and homogeneity?


texts: I saw an orange light in the sky which changed into flashing lights, what is this?

texts: It's Saturday 15th July 2017 and currently 00:44. I saw this strange object at around 00:35. I was looking out my window and spotted a bright orange light moving at a steady speed across the sky, it was definitely not a star, at first I thought it was maybe a plane and one of its engines was on fire but it was moving too fast to be a plane and it didn't appear to lose any speed or height as you might expect in that situation. Then I though it could be a landing plane since I live near an airport, again the same conclusion. I watched this bright orange light fly across the sky and occasionally dim as it went behind a cloud although it was still visible. It came out the first cloud and wntered the second, however when it came out of the second cloud it had changed into several dim red lights in a flat line, with the accasional bright flash of green to replace the red lights. It continued to fly over the airport and eventually got too far away to see anymore. I read up and though I could have seen a satellite but I've seen iridium flares before and they've been very different to this, this incident has left me confused and I'd love some insight as to what this might have been. Thanks :)


texts: Why are the spherical and cartesian galactic coordinates in the ATNF Pulsar Catalogue different?

texts:
I am trying to relate Galactic Latitude (b) and Longitude (l) (spherical coordinates) to galactic cartesian x,y,z coordinates in the ATNF pulsar catalog.

This query shows some sample data with G_l, G_b, XX, YY, and ZZ; excerpt:

------------------------------------------------------------------
#     NAME                     Gl     Gb     ZZ     XX     YY
                               (deg)  (deg)  (kpc)  (kpc)  (kpc)
------------------------------------------------------------------
1     J0002+6216    cwp+17     117.33 -0.07  -0.00  0.00   8.50
2     J0006+1834    cnt96      108.17 -42.98 -0.59  0.60   8.70
3     J0007+7303    aaa+09c    119.66 10.46  0.25   1.20   9.18

The description of the variables from the ATNF documentation are as follows:

GL:          Galactic longitude (degrees)
GB:          Galactic latitude (degrees)
[...]
ZZ:          Distance from the Galactic plane, based on Dist
XX:          X-Distance in X-Y-Z Galactic coordinate system (kpc)
YY:          Y-Distance in X-Y-Z Galactic coordinate system (kpc)

My understanding is that these variables should be related by the following equations:

tan(G_L) = YY/XX
tan(G_b) = XX/ZZ

However, when I test this assumption my calculated values are very different:

I have tried exploring the possibility that the x,y,z coordinate system may be oriented differently than I expect, but I can find no orientation that yields similar results for G_l or G_b:

Where could I have gone wrong? I feel like I am losing my mind not being able to convert these with simple trig.


texts: What is the temperature on surface of Pluto facing Sun?

texts:
According to wiki page, the mean temperature of Pluto is 44 degree K (-229 degree C). However, given that Pluto is tilted greatly and there is daylight for long periods on part of Pluto facing the Sun and also given that atmosphere of Pluto scatters light to a high degree, the surface temperature on that part of Pluto may be higher. The atmosphere of Pluto contains methane which is a powerful greenhouse gas, and that also could push up the temperature.

What can be the maximum temperature on part of Pluto facing the Sun? Does available temperature estimate take into account findings of New Horizons probe which flyby Pluto earlier this year? Thanks for your insight.


texts: Why is the TESS spacecraft finding fewer exoplanets than expected?

texts:
Judging from the Wikipedia page, I got the impression that TESS is underperforming, being expected to detect about 20000 candidate exoplanets in its primary two-year mission, but with only a few weeks left, it is now clear that it won't come close to that figure. I am just curious to know why that is.

From Astrobio.net's April 2018 article The Astrobiology Magazine Guide to TESS:

  “The number of Earth-sized and super-earth planets that TESS should find over the course of its two-year primary mission will be in the range of 500 to 1,000 new planets, and overall the number of planets that will be established is likely to be in excess of 20,000 all together,” enthuses MIT’s George Ricker, who is TESS’ Principal Investigator.

That is roughly 10 times greater than the updated total as of March 20, 2020 being in the 1700-1800 range according to NASA's status tracker.


texts: Please help explain this unknown light in the sky. Not ISS. Not a star or comet. Too slow

texts:
So a few nights ago I walked outside on my front lawn and sat down on the tailgate of my truck. I was facing north. I was looking down and towards the house across the street. Out of no where a bright light caught my eye. I looked up and didn't see anything. A couple of seconds later another light appeared, then disappeared. I pin pointed it to the left of the left of 'Kochab' star in 'Ursa Minor.' I was locked onto the sky at that point... So I kept watching and it kept happening. It was irregular. Zero regularity; it kept doing this. Bright flashes of light and subtle growing of light for a 20 minute period moving towards the north east. Randomly doing this with 2 seconds, sometimes with 5-7 second intervals. Even the type of flashing was irregular. Sometimes flashing following by flashing and other times growing light following by growing light. I've never seen anything like this in my life. I know what the space station looks like, I've seen it countless times as well as space junk. They're constant lights. And we all know what planes look like in the night sky. Constant red lights with regular blinking of white light. It was traveling in a straight path with little to no noticeable curvature. In no way could it have been reflecting light. No doubt it was projecting its own light.

Anything similar you've known or heard about? Please help explain this.


texts: Expansion again: where does the energy come from?

texts: Is it sensible to ask for the overall increase in potential energy when looking at the baryonic mass in the universe moving away from each other, that is, moving against the direction of the force towards a higher energy state in each others however weak gravitation field? I can't imagine that nature has the same take on this matter like a numerical simulation (a distance which it deems "large enough to make effects negligible so let's forget about it completely"), i.e. the energy can't be zero... where does it come from?


texts: Spectographically determine surface composition of exoplanet

texts:
We can determine the surface composition of a star by studying its spectrograph. If a exoplanet transits the star, then there is a slight change to the spectrograph, which lets us determine the composition of the planet's atmosphere.
When the planet just become visible from behind it's star, then the star's light will:-

come from the star (known composition)
travel through the planets's atmosphere (known composition)
reflect from the planet's surface (unknown composition)
travel through the planet's atmosphere again (known composition)

Is it possible (either currently, or theoretically) to use this method to determine the bulk surface composition of an exoplanet?
This might be very difficult to achieve as the light reflected from the planet will be swamped by direct light from the star.
How do astronomers determine the texture of an exoplanet? and Can visible wavelength spectroscopy study an exoplanet's chemical composition directly? might be releated


texts: Converting ecliptical to equatorial coordinates

texts:
When looking for a formula to convert polar ecliptic geocentric coordinates of an object to equatorial coordinates I find various sources that give these formulae (like Wikipedia):

Declination $δ = \arcsin(\cos ε \times \sin β + \sin ε \times \cos β \times \sin λ)$
Right ascension $α = \arctan((\cos ε \times \sin λ - \sin ε \times \tan β) / \cos λ)$

Where
β = ecliptic geocentric latitude
λ = ecliptic geocentric longitude
ε = obliquity of the ecliptic

But when applying these formulae I get results like in the following list where β = 0° and ε = 23.4°:

 λ       δ        α
  0   0.0000   0.0000
 45  16.3095  42.5443
 90  23.4000  90.0000
135  16.3095 -42.5443
180   0.0000  -0.0000
225 -16.3095  42.5443
270 -23.4000  90.0000
315 -16.3095 -42.5443
360  -0.0000  -0.0000

The values for declination seem good, but right ascension values seem to lack some sort of adjustment to the quadrant of the full circle (just a guess). But nowhere did I find any mentioning of this. Can you help? Thanks.


texts: Population density of stars within $13$ light-years of us

texts:
This item from Fox News refers to "over 600 stars that came within 13 light-years of the sun". Because of the past-tense "came", they could have meant stars that are not now within 13 light-years of us but were at some time in the past. But that is not made explicit in the article.

It is said that the nearest neighboring star is $4.3$ light-years from us. And $13$ light-years is about $3$ times that. So the volume within that distance of use is about $3^3=27$ times the volume within $4.3$ light-years of us. If there are $600$ stars within that larger volume, then the average number of stars within that smaller volume should be $600/27 \approx 22.$ But we don't have anywhere near $22$ stars within $4.3$ light-years of us.

So are we in a neighborhood that is unusually sparsely populated compared with what is typical in that larger volume?

Or is this about stars formerly near us that aren't now?

Or just confused journalists?


texts: Finding the diameter of a molecular cloud using period of intensity variation of hydroxyl maser

texts:
So I have this problem (which I  translated to English as accurately as could):

  When studying a compact molecular cloud with radiotelescope, at a wavelength of 18 cm, hydroxyl (OH) maser(ic) radiation was registered. Radiation intensity changes at about 5 min cycle. Evaluate the maximum diameter of the cloud.

Actually we learned very little about molecular clouds at high school and I have no idea how to solve this problem.

If the intensity change cycle is wave period, I can calculate wave velocity, which would be very low (as well as the frequency). I'm not asking for the answer, some hints would be enough. Thanks.


texts: Is there an alternative (potentially easier to understand) Euler diagram of the new solar system body classification?

texts:
note: This question is asking for a representation of the astronomical classification scheme now used for solar system bodies as established (presumably) by the Working Group for Planetary System Nomenclature (WGPSN) of the International Astronomical Union or IAU. While graphical layout is mentioned as part of the question's motivation, the question is not about layout or graphics.

The Euler diagram shown in this helpful answer is hard for me to understand, I think mostly due to graphical layout. The diagram shows the mathematical relationship between various sets (in this case solar system body classifications) so it could be drawn with widely differing choices of layout and still retain exactly the same mathematical relationships.

I am wondering if there is some attempt here to indicate relative numbers or ratios by the degree of overlap? Look closely at the small overlap of Minor Planets with Comets. Is that meant to reflect in some way a relatively small overlap quantitatively? Did you notice it took a while to even make sure you were seeing the overlap correctly? This style of layout requires the use of color perception to help identify which label is associated with which contour while simultaneously trying to follow one line in a groups of perfectly parallel lines.

Have other versions of this Euler diagram representation of solar system body classification been published? A link to one or even inclusion of the diagram in the answer itself would be greatly appreciated!

above: Example of an Euler diagram in this Wikipedia article, from here.

above: Example of a somewhat easier to use and understand Euler diagram, from here, used in the same Wikipedia article.


texts: How to find out which Messier objects are in the Kepler/K2 fields?

texts:
I am interested in figuring out which Messier globular clusters (plus 47 Tuc and Omega Cen) are in the Kepler/K2 fields, both fields that have already been observed and any that will be observed.

Poking around online I was able to figure out that Campaign 2 of K2 observed M4 and M80, but I would like to get a complete sample of globular clusters that have fallen or will fall into the Kepler/K2 fields, especially the Kepler field.  Are there online tools that could help me figure this out?


texts: If the planets only attract, then how can they stay in their orbit?

texts:
Gravity is related to mass, and gravity is an attracting force on every body. Then what happens when bodies only attract each other..without repulsion they may strike the sun. I think there must be a repulsive force.

Another question arises, if only attraction is there, then why does the moon rotate elliptically around the earth?

And Mars comes closer to earth after 3 years?


texts: What is the spectral power output of a typical galaxy?

texts:
I'm looking for a function which gives the typical isotropic spectral power of light emitted by an entire galaxy on the whole electromagnetic spectrum (in watts per frequency unit), as a function of frequency:
$$\tag{1}
\mathrm{d}\mathcal{P} = \mathcal{Q}(\omega) \, \mathrm{d}\omega = \; ?
$$
The total bolometric power emitted (in watts) is then
$$\tag{2}
\mathcal{P}_{\text{tot}} = \int_0^{\infty} \mathcal{Q}(\omega) \, \mathrm{d}\omega.
$$
What would be the function $\mathcal{Q}(\omega)$, for an "ideal" (theoretical) spherical galaxy?

As a candidate, how can we justify a function like the following one, where $\alpha$ and $\omega_0$ are two adjustable positive parameters, and $\mathcal{P}$ is a constant (the total bolometric power)?
$$\tag{3}
\mathcal{Q}(\omega) = \frac{\mathcal{P}}{\Gamma(\alpha + 1)} \; \Big( \, \frac{\alpha \, \omega}{\omega_0} \, \Big)^{\alpha} \; e^{-\, \alpha \, \omega / \omega_0} \; \frac{\alpha}{\omega_0}.
$$


texts: Computing the Sérsic profile of a galaxy from jpg images

texts:
I am trying to calculate the Sérsic profile of various galaxies from the SDSS based on the images provided by the galaxy zoo site. I am doing this as part of a kaggle competition on using machine learning to predict galaxy morphology. I have no chance of a high rank in this competition so I'm not hesitant to ask for help.

I used the R contourLines function to identify the isophotes of the galaxy and then fit ellipses to each isophote. This seemed to work well, the isophotes are almost always well fit by the ellipses and the ellipses are nearly concentric. Then letting I be the pixel intensity of an isophote and R be the length of the semi-major axis of the corresponding ellipse, I need to fit an equation of the form

log I(R) = log I_0 - k * R^(1/n)

The simple approach seemed to be to take the log of both sides and use OLS regression, so I fit a linear model in R of the form

log(log(I)) ~ log(R)

The resulting graphs showed a good fit but the resulting Sérsic indices n are almost always less than one and never as big as two. This doesn't seem right since indices of 4 or higher seem common in my reading. I don't get anywhere near 4 for an image of M87.

Possibly taking log log flattens things out too much and the index is not responsive enough. I tried using nls to work with just the log but it didn't move the indices much.

Is there any standard software or algorithm for computing the Sérsic index from an image? Are there reference images I can work from that would let me check if my algorithm is reasonable? Any recommendations on how to proceed would be welcome.

UPDATE: I have found the programs GALFIT and GIM2D which look like they might be useful. Any other software that is commonly used for this?


texts:
I took my telescope out tonight (I am new, just got the telescope a few days ago, it's my first one) and I was observing Mars, however all I see is just an bright orange circle, with no surface detail, regardless of magnification. The telescope I'm using is a 8in Newtonian, 1200mm focal length. Here's what I saw with a 8mm eyepiece and 2x Barlow:

Mars was about 32° above the horizon when I took this image.

Did I just do a poor job with collimation? Is the the observation angle? Please let me know!


texts: Semi-major axis, vs aphelion, vs mean orbital distance?

texts:
The website Swinburne University of Technology (AU) gives the definition of semi-major axis as:

  ...half of the longest diameter of an ellipse.

This website give the definition of semi-major axis as:

  Half of the major axis is called the semi-major axis, and the semi-major axis is also the average sun-planet distance.

But the average sun-planet distance should be the average of the semi-major and semi-minor axes. I've also seen the mean orbital distance described as the semi-major axis in many other places, not just the one cited, so that seems to be a fairly consistent definition, too. But both seem to be inconsistent; how could it be both?

And it also seems to be that the semi-major axis ought to be the same as the aphelion distance, and the semi-minor axis ought to be the same perihelion.

Obviously, the confusion must be mine, but I'm not sure where I'm getting things wrong.

Edit: It occurred to me shortly after posting this and shutting down my computer for the day that (as I'm sure 90% of this SE community knows) the semi-major and semi-minor axes are measured to the center of the ellipse, whereas the aphelion and perihelion are measured to the Sun. I think I'm starting to understand how the semi-major axis can be both the mean orbital radius with respect to the sun and the largest distance from the center of the ellipse.


texts: Does Doppler shift affect apparent pulsar frequency?

texts:
We know that redshift and blueshift is the result of the frequency of light waves reflected or emitted by an object lengthening or shortening due to the relative velocity of the observer moving away or toward the object. This is the Doppler effect.

However, it was stated in response to a question I asked on a sister site that the Doppler effect would also result in the apparent frequency of a pulsar's pulses being longer or shorter due to the relative velocity of the star. Given that the speed of light is constant regardless of the frame of reference (a concept I have a really hard time understanding by the way), is this correct?

My thinking is that it is incorrect. Using the analogy of a launcher lobbing medicine balls at me at a fixed rate, I can "see" the rate of launch by the frequency of when I get hit. However, if the launcher is moving away from me, the velocity of the balls lobbed at me will be slower (relative to my frame of reference), and thus will hit me with less frequency. However, if the launcher were to adjust its firing speed (i.e. speed of the ball relative to itself; frequency remains constant in this analogy) so that the balls always hit me at the same speed (thereby simulating the concept of the always-fixed speed of light), the time between them would not change.

So, given the always-constant speed of light, is it correct that a pulsar's pulse rate would appear to be different at different relative velocities?


texts: Why can you see the space station on some days but not on others?

texts:
In my area, from December 17th - 26th, you can see the space station every day, twice

But from December 27th - January 6th, you can only see it twice

My question is, why does it appear for almost the whole month of December, and then not appear for a few days? Is it based on my location? I live in Alberta, Canada if that helps.


texts: What causes horizontal and vertical lines coming out of pictures of stars?

texts: What causes the horizontal and vertical lines coming out of pictures of stars, instead of them simply appearing as circles? For example, this picture from Wikipedia: .


texts: Angles of sunrise and sunset viewed from a surface anyway oriented

texts:
I need to understand how to calculate the angles of the sunrise and sunset viewed from a surface oriented against the azimuth and  the horizon plane, and for any day of the year.

For example, I have a surface at this position 40° 43' 30.2"N 73° 59' 37.5"W, oriented towards north with an azimuth angle of 174°(O° is exactly the geometric south), and with a tilt angle of 90°,so the surface is vertical, or perpendicular to the horizon.

How could I calculate when it get the sun and when it doesn't during the day?
What are these angles against the azimuth axis?

In my example, sun should hit the surface when it rises, then it disappears, it reappears at the end of the day before of its set, mainly in the summer season. So in this case, I need to know four angles: -ws,w1,w2,ws, where -ws, and ws are the regular angle of rise and set, but how to calculate when sun disappears (w1), and appears again to the surface (w2)?

Do you know the complete formulas to these calculations?

Do you know any websites, or any web app to do exactly these type of calculations?

What about the regular case when the same surface in the same place and position but with its normal towards south?

I'm not very interested in the theory but in the practical calculation. I tried the formulas on this website (http://www.itacanet.org/the-sun-as-a-source-of-energy/part-3-calculating-solar-angles/) but it is not clear what happens if Azs=0°, that is when the surface is towards north. In the calculation of a and b for the formula 3.7 I get at the denominator sin(0)=0, times tan(beta=90°)=inf, if the surface is vertical!! How could I solve this problem?


texts:
I'm working with some data and I have some values named fluxmag0 and fluxmag0err that should correspond to the zero point values and error in flux units.

The values are around $10^{13}$ for fluxmag0 and $10^9$ for fluxmag0err

I need to have the zero point error in magnitude units. If I take $2.5\log_{10}(\mathrm{fluxmag0err})$ the values are huge so I suppose it's not the way to compute this.

How can I convert these values in terms of magnitudes?

Thanks a lot in advance for your help


texts: Calculating the equation of state parameter for matter

texts:
am a grad student in a cosmology class right now and I've got something of a head scratcher.
"Assuming the velocity of the atomic particles can be described by a Maxwellian distribution, calculate the equation of state parameter, ω , of the matter. Recall that the Universe is, to first order, 75% Hydrogen and 25% Helium."
What I'm unsure about is this: I know how to use a Maxwellian distribution to, for instance, get the total number density of nonrelativistic particles (I can do this piecemeal for protons, neutrons, electrons, and then figure out how those talk to one another for 75% Hydrogen and 25% Helium assumption... I think). I figure I can use each number density to get an energy density using ε = nE = nmc² and then sum the energy densities. Maybe this is a long and convoluted way, I'm not sure (I suspect it might be because I'm getting eqns for Maxwellian distributions out of chapter 8 and we're in chapter 5 of the text, and they do not appear in the instructors lectures either. Perhaps there's a much simpler way to do this I just haven't thought of).
In any case, say I do get the energy density of matter under the problem's given assumptions. How would I relate this to an equation of state parameter ω? I'm given a temperature, 3000 K, but that's the only information I'm given. I know that P = ωε (but then I'd be left with "well what's P.")
I'm missing something fundamental here with my conception. I know a temperature, I know I'm doing something with a Maxwellian distribution of velocities, and somehow I'm supposed to end up with ω, and it seems the professor is expecting me to go beyond the material of the notes or as far as we've gotten in the text to do so because I promise there is nothing covered in either that would do this.
Ideas?


texts: Have meteor showers been predicted for planets or solar-system bodies other than Earth? Have attempts been made to detect them?

texts:
Per the linked Q's and A's in Why do particles from a comet that result in meteor showers spread out mostly along the comet's orbit? and it's answer, meteor showers happen on Earth when it passes through the orbit of a comet.

There are a lot of known comet orbits in the solar system, and some of them likely intersect the orbits of planets other than Earth.

Question: Have meteor showers been predicted for planets or solar-system bodies other than Earth? Has there been any attempt to look for experimental evidence of these meteor showers, perhaps using radio or optical observations from spacecraft near some of these bodies?


texts: Almost unseeable light moving over nightsky. What is it?

texts: Sometimes when I look up at the sky at night, I can see something moving. It’s so small that I barely can see it, but I can’t see it when I look directly at it. I need to look beside it to see it. It slowly moves across the sky, but it switches sometimes directions. It sometimes disappears and after a while, it’s back. It’s not a strong light at all so it’s easy to miss.


texts:
Two Keplerian closed orbits always intersect:

Source: Astronomy SE answer by Flater
As seen from Earth, Venus appears larger than Mercury; this makes sense because Venus is both larger and closer than Mercury (as measured by minimal distance; by average distance all planets are equally distant), but can also be readily observed by comparing solar transits for either planet (or wait for 26 July 69,163, when both transit simultaneously).
Unless prevented by orbital resonance, that means that sometimes, Mercury should be eclipsed by Venus (occultation) when, seen from the Earth, Mercury is exactly behind Venus; and Mars should pass in front of the outer planets occasionally, but I'm not sure which body appears larger from Earth (Mars or Jupiter).
How often do occultations between planets occur?


texts: Why does the sky look like it has three suns in this video?

texts:
Check out @natalie9280’s Tweet:

  long story short, friday morning at around 6:40am i looked outside and i saw three suns.
  i checked to see if there was an eclipse and i could[n't] find anything.
  twitter help pls

  [VIDEO: three progressively brighter "suns," left to right in a closely spaced row]


texts:
Not a physicist just a simple guy, don't know much about physics as well. Just like to read things. I was reading about black holes and a question popped in my mind.

If I understand it correctly then the black hole is created by collapse of a super massive star at the end of its life. This star will have a finite mass. Then what leads to the black hole to have such massive gravity so that nothing can escape it (because if it was a star before with a finite mass then it would also have some finite gravity, right)?

And then they start absorbing more and more matter. So they grow bigger and bigger. Do black holes die?

Do all galaxies have a black hole at their center?

Thanks & Regards

Update-1

Found answer to the question, do black holes die?

Update-2

Found answer to the question, Do all galaxies have black hole in the center?

Posting the answer link here so anyone having this questions can find all answers in one place.


texts: How do we know what the biggest star is?

texts:
Kurzgesagt claims that the largest (observable) star in the universe would be Stephenson 2-18 which is in line with Wikipedia:

It is among the largest known stars, if not the largest, and one of the most luminous red supergiants, with an estimated radius around 2,150 times that of the Sun, which corresponds to a volume nearly 10 billion times bigger than the Sun.

Furthermore, Wikipedia says

The open cluster Stephenson 2 was discovered by American astronomer Charles Bruce Stephenson in 1990 in the data obtained by a deep infrared survey.

I stumbled upon that statement for two reasons:

The survey was made in 2010, so if we again would make a survey with newer, higher resolution data (which I assume we might have in the mean time), would we find other objects even bigger?
How are we sure that St2-18 was indeed the largest object within the given survey? Technically speaking, one would have to calculate at least the distance or lumnosity for each object of the survey?

In a nutshell: How sure are we (in percent) that St2-18 is right now, in 2020, still the largest observed star?
Related

What is the largest hydrogen-burning star?
Is there a theoretical maximum size limit for a star?


texts: Why is a meteorite impact on the moon visible as white light?

texts:
You can watch a video of a meteorite impacting the moon at the npr.org website.

Why does the impact produce white light?

I understand that the impact is like an explosion, but there is no air on the moon to burn material. If it was the result of superheating rock from the impact, then why does it fade so fast. White light emitted by heated rocks should last longer as it takes time to cool.

We can see meteorites breaking up in Earth's atmosphere as a streak of light, but there is no atmosphere on the moon.

So why isn't it just a big black puff of smoke?


texts: What would the sun look like from aphelion of The Goblin's orbit?

texts: I was amazed to find out about the discovery of The Goblin 2015 TG387 with an orbital period of more than 32 thousand years and a aphelion of more than 1955 AU! That is an astonishing distance. What would the sun look like from aphelion on The Goblin? Could a naked eye observer distinguish it from the rest of the sky?


texts: What's the farthest point humans have explored?

texts: By using powerful telescopes on land and space, what's the farthest point humans have explored or viewed?


texts: Is oxygen really the most abundant element on the surface of the Moon?

texts: I found this infographic that seems to say that oxygen is the most abundant element on the surface of the Moon. Is this really the case? If so, under what form is this oxygen?


texts: Is the Astronomy community still concerned about the lumpyness of matter distribution in the universe?

texts:
A decade or so ago, when I was still a science undergrad, one of the open questions in astrophysics was to explain the uneven distribution of galaxies in the observable universe.  That is, why did the universe look lumpy instead of evenly distributed.

Is this still an open question, and if so who is working on it?


texts: Did the earth form outside the solar system?

texts:
Why isn't it possible that the earth formed outside the solar system and got attracted later by the Sun?

I just need arguments to defeat this hypothesis.